The present invention relates to a signal processing circuit, more particularly to a signal reproducing circuit of a VTR or the like.
FIG. 1 shows an example of a conventional reproducing circuit of a home VTR (of VHS system, for example). Signals read from head 1A and 1B are supplied to pre-amplifiers 5A and 5B through rotary transformers 2A and 2B, resonance capacitors 3A and 3B, and damping resistors 4A and 4B, respectively. The head 1A, the rotary transformer 2A and the resonance capacitor 3A, and the head 1B, the rotary transformer 2B and the capacitor 3B are designed to have peaking frequencies near a white carrier (which is at 4.4 MHz in the VHS system) of an FM luminance signal. The damping resistors 4A and 4B serve to adjust peaking Q. FIG. 2 shows examples of a gain 11 and a group delay characteristic 12 of a prior art peaking circuit. In L-C resonance peaking, the group delay characteristic is not flat as shown in FIG. 2. A purpose of the peaking is to modify a frequency characteristic and match the heads 1A and 1B to the pre-amplifiers 5A and 5B to improve a noise figure. Outputs of the pre-amplifiers 5A and 5B are mixed by a switching circuit 13 actuated by a 30 Hz (in NTSC system) pulse train to produce a continuous signal, one portion of which is converted to the FM luminance signal a through a high-pass filter (HPF) 14 and the other portion of which is converted to a chroma signal b through a low-pass filter (LPF) 15. The FM luminance signal a is applied to an amplitude equalizer 16 which mainly equalizes an amplitude and one portion of an output of an amplitude equalizer 16 is supplied to a phase equalizer 17 which mainly equalizes a phase, a high-pass filter (HPF) 18 and a limiter 19 to produce a high frequency component c, and the other portion is applied to a low-pass filter (LPF) 20 and an amplifier circuit 21 to produce a low frequency component d. The high frequency component c and the low frequency component d are mixed by a mixer circuit 22. The equalizer 17, the HPF 18 and the limiter 19, and the LPF 20 and the amplifier circuit 21, and the mixer circuit 22 form a reversal prevention cricuit 10. When a reproduced output of the head decreases due to an increase of a spacing between the head and a tape, the high frequency component c is kept at a constant level because the amplitude thereof is clamped by the limiter 19, an amplitude of the low frequency component d decreases in proportion to a level of the reproduced output of the head. As a result, a mixed signal e of these components c and d is emphasized in a high frequency region. The higher is the frequency, the more is the decrease of the reproduced output of the head. Consequently, a reversal phenomenon in which a carrier of the FM luminance signal is lower than a level of a lower side band would normally occur. Such a reversal can be prevented by the above circuit configuration which emphasizes a high frequency region. The mixed signal e is applied to a limiter 23, a demodulator 24, a LPF 25 and a deemphasis circuit 26 to produce a luminance signal f, which is mixed with a chrominance (or chroma) signal g from a signal processing circuit 27 by a mixing circuit 28 to produce a video signal v.
The prior art circuit shown in FIG. 1 has the following disadvantages in the performance and the cost:
(1) The group delay characteristic is not flat (see FIG. 2) because the peaking is effected by the LC resonance circuit.
(2) The compensation by the equalizer 16 provided to compensate for the amplitude characteristic and the group delay characteristic is insufficient.
(3) The HPF 14 is necessary to separate the luminance signal.
(4) The phase equalizer 17 for the high frequency component c and the low frequency component d is necessary in the reversal prevention circuit.
FIG. 3 shows an example of a PAL signal reproducing circuit in a prior art home video system. Reproduced signals from heads 1A and 1B are supplied to pre-amplifiers 5A and 5B through rotary transformers 2A and 2B, resonance capacitors 3A and 3B and damping resistors 4A and 4B, respectively. The head 1A, the rotary transformer 2A and the resonance capacitor 3A, and the head 1B, the rotary transformer 2B and the resonance capacitor 3B are both designed to have peaking frequencies near a white carrier (4.4 MHz in the VHS system) of an FM luminance signal. The damping resistors 4A and 4B serve to adjust peaking Q. Outputs of the pre-amplifiers 5A and 5B are mixed by a switching circuit 13 actuated by a head switching pulse train (at 30 Hz in the NTSC system and 25 Hz in the PAL system) to produce a continuous signal, one portion of which is supplied to a HPF 14 to produce a luminance signal a and the other portion of which is supplied to a LPF 15 to produce a chroma signal b (at 627 KHz in the VHS system). The FM luminance signal a is supplied to an FM drop-out compensation circuit (FMDOC) 34 comprising a switch 32 and a 1H-delay line (where 1H is one horizontal scan period), a phase equalizer 35, a limiter 36, a demodulator 37, a LPF 38 and a deemphasis circuit 39 to produce a luminance signal f. On the other hand, the chroma signal b from the LPF 15 is supplied to an up-converter 29 to produce a chroma signal c of 4.43 MHz, which is supplied to a comb filter 27 comprising a 2H-delay line 30 and a subtraction circuit 31 to produce a chroma signal e having a reduced influence by adjacent tracks on a tape. The chroma signal e is mixed with the luminance signal f by a mixer circuit 28 to produce a video signal v.
FIG. 4 shows time relations between luminance signals 70 and chroma signals 71, in which (A) shows a time relation between the luminance signal f and the chroma signal b of FIG. 1, (B) shows a time relation between the luminance signal f and the 2H-delayed chroma signal d, and (C) shows a time relation between the luminance signal f and the chroma signal e at the output of the comb filter 27. In the (A), a chroma center (center of lateral symmetry) is at 72 between C2 and C3, and a corresponding position of the luminance signal is between Y3 and Y4. In the (C), however, the chroma center is at 73 between Y4 and Y5 of the luminance signal and it is delayed by 1H relative to the (A). On a TV screen, it corresponds to that the chroma signal is shifted down by 1H relative to the luminance signal for each recording/reproducing operation of the VTR. When dubbing is repeated by the VTR, the chroma center is shifted down by 1H for each dubbing and a quality of image is significantly degraded.